1. Field of the Invention
The present invention relates to a color plasma display panel device and, more particularly, to a device for compensating for the image distortion of a plasma display panel by reducing the luminance difference between gray scale signals when the gray scales are realized with a subfield driving method.
2. Discussion of Related Art
According to the development and spread of data processing systems in the modern communications society, data display devices are of more importance and increasingly developed in their various types.
A CRT (Cathode Ray Tube) has been most widely used as a data display device, but it requires a large screen and large operating voltage. Also, when it displays straight lines, they appear curved. Because of these problems with the conventional CRTs, much research and development is being made on the flat display panel devices of all types having a matrix structure that will suit the purpose of recent tendencies to have a full-sized and flat screen.
The flat display panel devices have a flat matrix structure to display stable images thereon without any distortion or color blots. They are widely applicable such as full-sized televisions, wall-attached televisions and portable computers due to their thin shapes and low operating voltages.
The flat display panel device comprises various luminescent elements, i.e., PDP (Plasma Display Panel), ELD (Electroluminescent Display) and LED (Light Emitting Diode), or non-luminescent elements, i.e., LCD (Liquid Crystal Display), ECD (Electrochromic Display), DMD (Digital Mirror Device), AMD (Actuated Mirror Device) and GLV (Grating Light Valve).
A flat display panel device that comprises a PDP as one of the luminescent elements produces motion or still pictures by utilizing the gas discharge occurring in the PDP.
In the PDP as shown in FIG. 1, N first sustain electrode lines X.sub.1, X.sub.2, . . . , X.sub.N-1, X.sub.N and N second sustain electrode lines Y.sub.1, Y.sub.2, . . . , Y.sub.N-1, Y.sub.N are arranged in parallel at regular intervals, and M RGB address electrode lines R.sub.1, G.sub.1, B.sub.1, R.sub.2, G.sub.2, B.sub.2, . . . , R.sub.N-1, G.sub.N-1, B.sub.N-1, R.sub.N, G.sub.N, B.sub.N in parallel at regular intervals are deposited at right angles to the first and second sustain electrode lines X.sub.1 .about.X.sub.N, Y.sub.1 .about.Y.sub.N.
Discharge cells defined by respective intersections of the two lines on the screen have discharge gaps of the same size.
An image is displayed on the PDP when a gas discharge occurs selectively in each cell which is energized by the address and sustain electrodes arranged on the front and back glass plates. Forming one element of a matrix display can be realized by the production of plasma gas discharge and maintenance discharge between the address and sustain electrodes.
As a conventional method of driving the plasma display panel device, subfield driving methods are disclosed in Japanese Unexamined Patent Publication No. 4-195188, Japanese Patent Application No. 4-0340498, U.S. Pat. No. 5,436,634, and U.S. Pat. No. 5,446,344). In the subfield driving method, one frame is divided into x subfields to realize 2.sup.x shades of gray. The relative ratio of the luminances of the subfields is 1:2:4:8:16:32:64: . . . . Thus the combination of several subfields forms a display whose elements correspond to the gray scales 0.about.2.sup.x -1 in cells.
In FIGS. 2 and 3, a frame is divided into eight subfields SF1 to SF8. The ratio of the luminances of the subfields SF1 to SF8 is 1:2:4:16:32:64:128 to realize 256 shades of gray corresponding to the gray scales 0.about.255 (usually, represented by 8 bits, D.sub.7 .about.D.sub.0).
To drive the first subfield SF1, a gray scale signal of D.sub.0 bit is applied to each cell so as to determine whether a discharge is carried out in the cell or not. If driving the second, third, fourth, fifth, sixth, seventh and eighth subfields SF2 to SF8, gray scale signals of D.sub.1, D.sub.2, D.sub.3, D.sub.4, D.sub.5, D.sub.6 and D.sub.7 bits are applied to the cells, carrying out a discharge in a selected cell and maintaining it for a predetermined period of time to produce an image.
The plasma display device using the subfield driving method of the prior art is described as follows.
FIG. 4 is a schematic view showing the construction of the conventional plasma display device. Referring to FIG. 4, the plasma display device comprises: a PDP 1 having a plurality of address electrode lines, and first and second sustain electrode lines; RGB image signals that will be fed into the plurality of address electrode lines according to the signals externally supplied; first and second sustain pulses that will be fed into the first and second sustain electrodes; a control unit 2 for generating all kinds of control signals; first and second address drivers 3 and 3' for supplying the RGB image signals to the plurality of address electrode lines according to the control signal of the control unit 2; and first and second sustain drivers 4 and 5 for supplying the first and second sustain pulses to the plurality of first and second sustain electrodes according to the control signal.
As shown in FIG. 4, the first and second sustain pulses have the pulse difference of 180.degree. with each other. When the first and second sustain pulses are applied to the first and second electrodes of the PDP 1, discharges are carried out between the first and second sustain electrodes at each rising edge of the respective pulses. Thus maintenance discharges occur between the address electrode and the first sustain electrode, to maintain the discharges.
The luminance of each gray scale signal is determined depending on the number of the first and second sustain pulses when realizing the gray scale by means of the subfield method to drive the plasma display device.
The process for producing a motion or still picture on the plasma display device according to the subfield driving method is described as follows.
Referring to FIG. 4, the control unit 2, and first and second sustain drivers 4 and 5 supply given pulses to the first and second sustain electrode lines X.sub.1 .about.X.sub.N, Y.sub.1 .about.Y.sub.N, thus erasing the discharges in all cells to prevent the respective cells from being affected by the prior discharges.
The first and second sustain drivers 4 and 5 supply scan pulses to the first and second sustain electrode lines X.sub.1 .about.X.sub.N, Y.sub.1 .about.Y.sub.N, and the first and second address drivers 3 and 3' supply a RGB image signal of one bit to the designated one of the address electrode lines R.sub.1 to R.sub.N. The fluorescent material coated on a designated cell that receives the 1-bit data (a high pulse) is excited to emit a light.
When the control unit 2, and first and second sustain drivers 4 and 5 supply the number of first and second sustain pulses corresponding to the first subfield SF1 to the first and second sustain electrode lines X.sub.1 .about.X.sub.N, Y.sub.1 .about.Y.sub.N, the luminescence in a designated cell is maintained for a period of time corresponding to the first subfield SF1.
The steps of erasing the discharges in all cells, supplying the RGB image signals, and supplying the first and second sustain pulses are repeated for the rest of the subfields SF2 to SF8, finally producing an image on the PDP 1.
When the gray scale is realized by the subfield driving method as described above, the luminance of the RGB image signals applied to each cell depends on the number of first and second sustain pulses applied to a plurality of the subfields constituting one frame. The color of one pixel is determined by the combination of the luminances of the RGB image signals.
Various colors are realized in one pixel by the combination of the red, blue and green visible lights of three RGB cells energized according to the RGB image signals (which are of the same bits as the number of the subfields) supplied to the address electrode lines R.sub.a, G.sub.a, B.sub.a of the RGB cells constituting one pixel for one frame.
Referring to FIG. 5, according to the subfield driving method as described above, various gray scale signals can be displayed by the combination of the subfields SF1, SF2, SF3, . . . each of which corresponds to the number of first and second sustain pulses proportional to the ratio of luminance (1:2:4:8:16:32:64:128: . . . ).
For example, after one frame is divided into eight subfields SF1 to SF8 to realize 256 (=2.sup.8) shades of gray, the number of first and second sustain pulses that is proportional to the relative ratio of the luminances of the subfields SF1 to SF8 (1:2:4:16:32:64:128) are supplied to each subfield. With the combination of the eight subfields SF1 to SF8, the gray scale signals 0 to 255 can be displayed.
The process of displaying a gray scale signal, i.e., the gray scale signal 127 in one cell is described as follows. When driving the subfields SF1 to SF7, the pulse is supplied to the address electrode of a cell concerned so as to discharge the cell with the pulse supplied to the first sustain electrode and to maintain the discharge of the cell for a corresponding period of time. On driving the subfield SF8, the pulse is not supplied to the cell concerned, thus interrupting the discharge of the cell.
The cell supplied with the gray scale signal 127 is turned on seven times through the subfields SF1 to SF7, only to have the luminance proportional to the relative ratio of the luminance 1+2+4+8+16+32+64=127.
However, the quality of the image may be deteriorated such as an image distortion caused when adjacent gray scale signals are supplied in succession to one screen, for the luminance difference between the successive gray scale signals are too large due to the discharge mechanism by which the luminance varies right after the startup and just prior to the completion of the discharge.
For example, consider a gray scale signal 7 (1+2+4) that is realized with three times (or subfields) of discharge over a short period of time, relative to an adjacent gray scale signal 8 that is realized with one time (or subfields) of discharge over a long period of time. This results in a large luminance difference between the gray scale signals 7 and 8, and thus causes the image distortion when the two signals are successively supplied.
Further, the most severe image distortion may take place when the gray scale signal 256 is supplied with the gray scale signals 127 and 128 in succession on one screen.